1. Field of the Invention
This invention relates to a dry process for gasifying coal or other carbonaceous material in a rotary kiln. In particular this invention relates to improvements in such a process as may be carried out in rotary kilns having gas outlets on both ends of the kiln and air-steam admitting ports extending radially through the shell of the kiln to rotate with the kiln and pass beneath a bed of coal in the kiln.
2. Description of the Prior Art
Coal gasification processes carried out in an inclined rotary kiln are known to have several advantages. Rotary kilns for coal gasification can (1) be built to very large scale; (2) throughput can be easily controlled by adjusting rotational speed; (3) feed material need not be screened to particular sizes since a rotary kiln handles chunks and very small fines; (4) ample room can be provided to accommodate swelling of caking coals; and (5) constant mixing of the coal charge due to kiln rotation provides substantially homogeneous temperatures within the bed at each location along the longitudinal axis of the kiln.
Coal gasification processes carried out in an inclined rotary kiln have been known at least since U.S. Pat. No. 247,322 to F. Egner dated Sept. 20, 1881; and such a process was disclosed as being carried out in a rotary kiln having gas ports extending radially therethrough in French Pat. No. 149,049 of 1882. U.S. Pat. No. 1,121,906 to G. E. Downs dated Dec. 22, 1914 discloses a ported rotary kiln sealed for operation with pressurized air or other gas or vapor supplied through the ports in the kiln shell.
The first suggestion of a process by which two fuel gas streams of different composition may be produced in a single system, is believed to be found in U.S. Pat. No. 1,159,675 to J. W. Hornsey dated Nov. 9, 1915. The process of this 1915 Hornsey patent utilizes three rotary kilns, a first kiln for drying and preheating coal, a second kiln for educting gases at about 900.degree. F. which contain condensible volatile matter (referred to as "coal gas"), and a third kiln for educting gases from coal at over 1,600.degree. F. in an atmosphere of steam, with such gases substantially free of condensible volatile matter (and such gases being referred to as "water gas").
U.S. Pat. No. 1,214,164 to J. W. Hornsey dated Jan. 30, 1917 is perhaps the first disclosure of a coal gasifying rotary kiln having gas outlets on both ends of the kiln. This 1917 Hornsey patent discloses two processes that can be carried out with such a kiln. The first described process is for producing one gas, water gas, from a charge material that has been previously coked. The description of this water gas process states that steam input and water gas output may be from either end of the kiln or in any conbination. The second process described in this patent is a process for producing two gas streams, coal gas and water gas, in a single rotary kiln, with coal gas being discharged from the coal inlet end of the rotary kiln and the water gas being discharged from the ash discharge end of the kiln.
Hornsey patents U.S. Pat. No. 1,267,410 dated May 28, 1918 and U.S. Pat. No. 1,270,949 dated July 2, 1918 again both describe two processes. The first process described in each of these patents is a process in which gas is drawn off from the coal inlet end of a rotary kiln and the input of steam is increased or decreased to control gas temperature and gas composition. The second process described in each of these patents is a process in which a tar-free gas is produced by controlling an input of air to promote combustion, and this gas is discharged from the solids discharge end of the kiln. That process is described as providing air in an amount sufficient to convert carbon monoxide to carbon dioxide and produce heat in an amount that the tars are "raised to such a high temperature that they are converted into fixed gases." Such high temperatures are not specifically described therein, but it is known that to so convert such tars and at the same time operate the process efficiently (i.e., with a substantially complete conversion of coal to gas) the temperatures within the gasifier would be of the order of 2,500.degree. F. to 3,300.degree. F. Such operation has two disadvantages. First, such high temperatures are very hostile to the rotary kiln and its lining. Secondly, by such operation much of the energy contained in the coal is converted to unrecoverable sensible heat rather than the more desired conversion of the energy in the coal to latent chemical energy in a combustible fuel gas releasable as thermal energy upon combustion of the fuel gas. The conversion of much of the energy in the coal to sensible heat rather than latent chemical energy in the fuel gas in this Hornsey air-blown rotary kiln process, means that the fuel gas produced, while free of tars, would have a very low latent chemical heating value and much lower than fuel gas produced by the present invention. If, on the other hand, this Hornsey process is operated to convert the tars to gases at significantly lower temperatures (e.g., about 1,900.degree.-2,000.degree. F.) the latent chemical heating value of the generated gas will be higher per unit volume of gas but the total chemical heating values (BTU's) educted per ton of coal will be much lower than that for the present invention.
As will appear from the description of the present invention to follow, the present invention may be considered as being an improvement to the processes disclosed in the aforesaid 1917 and 1918 patents to Hornsey.
Before proceeding with a description of the present invention, three other patents will be reviewed that relate to gasifying coal in a rotary kiln, and which were granted after the aforesaid Hornsey patents.
U.S. Pat. No. 1,916,900 to J. N. Vandegrift et al. dated July 4, 1933 discloses a rotary kiln with gas outlets on both ends. However, the process described in this patent is a relatively low temperature process (does not exceed 1,100.degree. F.) to recover "coked residues" and "to convert lumps of semi-bituminous and cannel-coals and some bituminous coals into lumps of anthracite", and this process is not, therefore, designed to maximize eduction of fuel gas from the feed material.
U.S. Pat. No. 3,990,865 to A. Cybriwsky and G. T. Petersen, dated Nov. 9, 1976, discloses a rotary coal gasification kiln with radial ports carried by the rotating kiln shell for admitting air and steam to the interior of the kiln, either when the ports are passing beneath the bed of coal particles within the kiln or continuously as the ports are carried in this circular orbit around the bed of coal within the kiln.
U.S. Pat. No. 4,214,707 to T. W. Flaherty dated July 29, 1980 discloses an arrangement for fluid supply pipes, valves, ports and nozzles for ports, for injecting air and steam into a coal gasification kiln such as is described in Cybriwsky and Petersen U.S. Pat. No. 3,990,865 and or in the description to follow with regard to the present invention.